I. Field of the Invention
The present invention relates generally to high frequency transmitters and, more particularly, to a method and apparatus for modulating the transmission signal utilizing phase shifters.
II. Description of Related Art
For electromagnetic radiation, the length of an antenna radiator is inversely proportional to the frequency of the electromagnetic transmission. Consequently, it is very difficult to transmit low frequency signals, such as voice or video, as electromagnetic waves at their original frequencies. In order to do so, would render the necessary antenna radiators unacceptably long.
Consequently, in communication radio, a low frequency signal, also known as a baseband signal, is typically upconverted in frequency to the carrier frequency. For example, in AM radio voice signals having a bandwidth of about 5 kilohertz are typically upconverted to 531-1611 kilohertz for commercial medium wave AM radio, or to 2.5-26.1 megahertz for shortwave AM radio. By upconverting the low frequency of the voice signal, transmission of the radio signal can be accomplished with a practical sized antenna.
The process of upconversion is done through the use of a mixer which mixes a desired baseband signal, known as the IF or intermediate frequency signal, with a high frequency signal known as the LO (local oscillator) signal. The mixer, however, does not simply add the LO and IF signals together. Instead, the output from the mixer contains both the sum and the difference of the LO and IF frequencies, as well as the LO signal. For example, assuming an LO signal of 10 megahertz and an IF signal of 5 kilohertz, the output from the mixer would contain not only the LO signal of 10 megahertz, but also two sideband signals that are modulated by the IF frequency. These signals would be centered at 9.995 megahertz and at 10.005 megahertz.
Although it is possible to amplify both the LO signal and both sideband signals for transmission from the appropriate antenna, to do so is very power inefficient. This power inefficiency arises since each sideband signal contains the same modulation information and the LO signal contains no useful information at all. Consequently, in order to increase the power efficiency of the transmitter, it is necessary to remove one of the sideband signals as well as the LO signal prior to amplification and transmission. However, this requires very narrow band filters to filter out the undesired signal. The remaining signal is then amplified and transmitted. Such filters, however, add distortion to the output signal.
While the previously known practice of using upconversion optionally followed by filtering of the upconverted signal has proven effective for relatively low frequency radio transmitters, i.e. radio transmitters transmitting at a frequency of less than 10 gigahertz, such radio communications transmitters have proven impractical for high frequency radio transmitters, i.e. radio transmitters in the range of 0.1 to 10 terahertz, for a number of different reasons.
First, solid-state frequency sources at such high frequencies, i.e. greater than 100 gigahertz, are necessarily low powered, typically generating 1 watt or less in power. Furthermore, for low distortion, the LO signal power must be at least 10 times the power of the IF signal desired to be upconverted and ultimately transmitted. Consequently, with an LO signal source having a 1 watt output and used as a carrier wave input to the radio mixer, the input from the RF signal to be modulated cannot be greater than 0.1 watt. Otherwise, excessive distortion of the modulated sideband signal would occur.
In order to improve the overall power efficiency of the transmitter, the carrier signal and one sideband should be removed from the output signal from the RF mixer. However, to do so requires a very narrow band filter. Such narrow band filters, particularly at high frequencies, are not only lossy but do not provide flat group delay. Such narrow band filters also distort the signal and this distortion becomes worse as the filter bandwidth becomes narrower. All these factors thus result in distortion of the signal desired for transmission as well as a very low output power for the modulated signal. This, in turn, requires excessive amplification in order to reach an acceptable power level, e.g. a few watts, for practical applications.
For all the foregoing reasons, the use of RF mixers which mix IF sources with the LO mixer input, even when followed by frequency multipliers which also add distortion, results in excessive distortion and power loss for the RF signal.